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alkene (ălˈkēn), any of a group of aliphatic hydrocarbons whose molecules contain one or more carbon-carbon double bonds (see chemical bond). Alkenes with only one double bond have the general formula CnH2n. In the International Union of Pure and Applied Chemistry (IUPAC) system of chemical nomenclature, the name of an alkene is derived from the name of the corresponding alkane by replacing the -ane alkane suffix with -ene and, if necessary, adding a prefix to indicate the location of the double bond in the molecule. The IUPAC name of the simplest alkene, H2C=CH2, is ethene, which is derived from ethane. Propene is related to propane. Two alkenes, 1-butene and 2-butene, are related to butane; these two compounds, which differ in the location of the double bond in their molecules, are structural isomers. In addition to these IUPAC names, many of the alkenes have common names, e.g., ethene is called ethylene and propene propylene. The alkenes as a group are sometimes called the ethylene series. Since the carbon-carbon double bond is sometimes called an olefinic linkage, the alkenes are sometimes called the olefins. Many of the reactions in which alkenes take part involve the cleavage of half the carbon-carbon double bond and subsequent formation of two single bonds, one to each of the adjacent carbon atoms. Such reactions include hydrogenation, with the formation of an alkane, and hydration, with the formation of an alcohol.